Analog signals can be digitized using real-time sampling technology or equivalent time sampling technology. Real-time sampling technology typically samples an entire waveform on each trigger event so that a large number of data points are captured. Real-time sampling can be triggered by an external trigger signal or based on a feature of the data itself such as when waveform's amplitude reaches a certain threshold. Real time sampling is often limited by the sampling rate of the capturing device, as faithful reproduction of the waveform requires a sampling rate that is at least twice the highest frequency of the waveform. For real-time sampling technology, an analog signal is digitized in two stages: discretization and quantization. Discretization is a process of dividing the signal into intervals of time, and each interval is represented by a single measurement of amplitude. During quantization, each amplitude measurement may be approximated by a value from a finite set.
Equivalent time sampling measures the instantaneous amplitude of the waveform at the sampling instant. In contrast to real-time sampling, in conventional equivalent time sampling techniques an input signal is sampled once per trigger, but this process is repeated for each delayed version of the trigger to capture additional samples from the multiple triggers. In particular, the next time a trigger is generated, a small delay is added and another sample is taken. Conventional equivalent time sampling technology uses an external clock to trigger an analog-to-digital converter to acquire a single sample after each transmitted pulse. Such a design causes the performance of the equivalent time sampling system to be limited by the characteristics of the clock.